Acetal or Delrin – Does it Really Matter Which One You Use?
While these two polymers share many similar characteristics, there are definite and significant differences that can dictate your choice of material.
Acetal is the common name for a family of thermoplastics with the chemical name "PolyOxyMethylene", or POM. Acetal is available in two general types of resins: Copolymer acetal (POM-C), and homopolymer acetal (POM-H); commonly called Delrin®. Consequently, both are acetal polymers, but each type of acetal has its own set of advantages and disadvantages.
Acetal Copolymer Considered
The copolymer grade offers excellent performance, but at a slightly lower cost than Delrin®. Acetal copolymer offers consistent properties throughout the shape since it is free of centerline porosity. Its low stress levels and high strength will assure flatness and dimensional stability up to a maximum continuous service temperature of 80°C (180°F).
Copolymer acetal (POM-C)grades are also FDA, USDA, NSF, and 3-A Dairy compliant, which means it is great for food compatible uses. In addition, acetal copolymers are available in a wide variety of colors.
Acetal Homopolymer or Delrin®
Delrin® acetal homopolymer (POM-H) offers slightly higher mechanical properties than acetal copolymer, but can often contain a low density center – also known as "center line porosity” – especially in large cross-sections. Delrin® also gives slightly less chemical resistance than copolymer acetal. However, Delrin® is ideal for applications such as small diameter, thin walled bushings that would benefit from the additional strength and rigidity of homopolymer acetal.
How they compare
Some advantages of copolymer acetal over homopolymer acetal (Delrin®) are:
- Better dimensional stability due to its lower level of crystallinity
- Better resistance to hot water and strong caustics, or high pH (basic) solutions
- Lower coefficient of friction and better impact and wear properties, especially in wet or moist environments
Some advantages of homopolymer acetal (Delrin®) over copolymer acetal are:
- Greater stiffness, with higher flexural modulus at room and elevated temperature applications
- Slightly higher tensile and impact strength at room temperatures
- Slightly harder, thus giving the homopolymer acetals a lower coefficient of friction
What it all means for you
The fact is that in most applications, Delrin® and acetal copolymer can be interchanged because many material properties are within approximately 10% of each other. The most significant property difference between Delrin® and copolymer acetal relates to what is commonly known as centerline porosity – an inherent characteristic of Delrin®.
Centerline porosity is most prominent in thick slab and large diameter rod stock. It can be seen quite easily around the center portion of a rod, which extends down the entire length. In sheet stock, porosity shows up as a line along the center of each cut edge. With some stock, the slab may even look as if it is laminated or glued together.
The cause of the porosity is shrinkage. During the extrusion process, the outside of the shape cools before the interior. As the interior material cools, there is a corresponding reduction of volume. Since the volume change of the shape is restricted due to the solidified skin, voids form to compensate for the loss of interior volume.
Excessive centerline porosity is undesirable for the following reasons:
- Aesthetic -inconsistent color appearance in finished parts.
- Compromises structural integrity.
- Presents potential routes for leakage of gas and liquids.
- Provides areas where the bacteria can grow in food processing applications.
A last word
Other considerations are that Delrin® has 18% better tensile strength, is 16% more rigid, 12% harder, and 8% better in terms of operating temperatures. Acetal is food compatible whereas Delrin® is not. Acetal is also resistant to the cleaning agents used in food industries.
They are both incredibly difficult to bond and are both very “slippery” plastics. However, they are both easy to machine, have very low moisture absorption, and give a very nice machined surface finish. Acetal is approximately 29% cheaper than Delrin® and has a slightly greater range of sizes available.